1. Field of the Invention
The present invention relates to multi-shaft extruder kneading discs and kneading blocks for kneading material in a multi-shaft extruder and to a multi-shaft extruder containing the same. More particularly, the present invention relates to kneading discs and kneading blocks wherein each of the kneading discs has a plurality of lobes with at least one of the lobes having a radius less than another lobe.
2. Background Art
Kneading discs for twin-screw extruders are well known. Generally, the kneading discs have 2 to 3 kneading lobes the length of the bushing. The crests of the kneading lobes are generally flat in the radial direction, but do have an arcuate shape in the circumferential direction. These prior art kneading discs are usually manufactured such that side by side kneading discs interact with each other when mounted in a twin screw extruder as kneading blocks. The number of discs in a kneading block are generally 4, 5 and 7. That is, the typical two diametrically opposed lobe kneading discs has the discs oriented at 60 degrees to each other in a 4 disc block; at 45 degrees or 90 degrees to each other in a 5 disc block, and at 30 degrees to each other in a 7 disc block.
The kneading discs have an upper face and a parallel lower face which are substantially mirror images of each other. Lateral side walls join the upper and lower faces. The discs generally have an annular central region with two or three lobes radially extending from the central region. The crests of the lobes are generally truncated. The kneading blocks generally have the discs arranged thereon so that they are in balance.
By increasing torque capacity and available rpm (speed), the capacity of an extrusion production line has been increased without increasing extruder size (diameter). However, an increase in speed has resulted in higher material temperature. (For a more detailed commentary on this issue, please refer to the '97 Antic paper "Understanding High Rate and High RPM Compounding on co-Rotating Twin-Screw Extruders.) As rpm is increased, the material will, at some point, reach a temperature where the polymer will start to degrade.
Temperature is generated by the deformation of material as a consequence of energy input. This energy input results from the shear generated as: (1) two discs (each disc is on a different screw shaft) pass each other going in opposite directions, (2) the lobe crest of one element disc wipes the root of the other, and (3) the lobe crests of two discs approach each other in the area of the screw channel known as the apex region. In the apex, an open surface of a small but not inconsequential area is reduced to virtually zero area during the disc rotation. In the standard full diameter kneading block element, all three of these actions, as well as the pushing of material in a circumferential motion by the disc lobe crest, contribute to material deformation and an associated temperature rise.